Electromagnetic apparatus for recording sound



5 Sheets-Sheet 1 {u I H-H' ilk": 4

M r h 15 1927.

a c A. F. sYKEs ELECTROMAGNETIC APPARATUS FOR RECORDING SOUND Filed Aug. 16, 1926 192 March 7 A A. F. SYKES I ELECTROMAGNETIC APPARATUS FOR RECORDING SOUND Filed Aug. 16, 1926 5 Sheets-Sheet 2 March 15 1927.

A. F. SYKES ELECTROMAGNETIC APPARATUS FOR RECORDING SOUND Filed Aug. 16, 1926 5 Sheets-Sheet '3 March 15, F). SYKES ELECTROMAGNETIC APPARATUS FOR RECORDING SOUNDQ Filed Aug. 16, 1926 5 Sheets-Sheet 4 March 15, 1927. 1,621,180

A.F.SYKE$ ELECTROMAGNETIC APPARATUS FOR RECORDING SOUND Filed Aug. 16, 1926. 5 Sheets-Sheet 5 Patented Mar. 15, 1927.

UNITED STATES eATs T OFFICE.

Amman rmcls sizes. or nawnamm'r, lineman.

ELECTROMAGNETIC APPARATUS FOR RECORDING SOUND.

Application filed August 16, 1928, Serial No. 129,812, and in Great Britain 111115412, 1925.

This invention constitutes anim rovecompensating conductor are carriedv on a ment in or modificationof my invention as floating weight so that the electromagnet 55 claimed in British'specification No. 210956.

which is of considerable mass, is fixed. A

According to that specification sound is re-ieonvenient method of applying the invention 6 corded in electrical fashion'by means of an jgnsists in forming the compensating con;

electromagnetic engraving device of oscillo graphic type comprising a working conductor or conductors consisting of 'one or more strips of metal generally in tension and located in a magnetic field. The vibration of the strip or st-rips'in response to forces derived from electric currents of a telephonic nature is utilized to produce a sound record.

\ Elastic control of the strip orstrips is-the chief feature of the invention and the mini- .mum of'm'ass is a desideratum. In some forms ofsuch apparatus considerable forces are brought to bear on the framework or floating weights and itzis-the object .of the present invention to relieve the frameworkor floating weights from such forces as are liable to set up undesirable resonance effects. The invention more particularly applies to the case in which the air gap of the field 5 magnet is on one side of the wax tablet or record blank. With such apparatus it is possible if desired to obtain aperiodic response over a wide range of frequency. The.

first part of my invention relates then to means for annullingthe electro-mechanlcal effect on the structure or on a floating beam. Thesecond part is concerned with means for obtaining a wide range of action as regards amplitude of vibration in a simple form. The third part of my invention 1s connected 'with the concentration of the resistance of the working conductor and also of the magnetic field. The fourth part of my invention involves considerations perta ning to damping the vibration of the working conductor.

According to the first'and'main part of my invention I proyide a further conductor adjacent to the working conductor i'mmersedin substantially the same field, or in a similar I produce electr'o-mag-.

field, and adapted to netic forces substantial y equal and opposlte to those produced. by the working conductor. This subsidiary or compensating conductor may be fixed to the structure or it may be able to vibrate either in free relation to the structure or damped with respect thereto.

Preferably the working conductor and the 'tlfictor and the acting conductor. from one strip or wire by bendlng it back on itselfi in U or V form; a special case is where this combination is realized in the shape of a solenoidal winding of many turns forming'a.

high resistance recording-element. The invention further consists in the improved recorder and the subordinate details and method of working hereinafter described in connection with Figures 1 to 13 of the acductor due to its ohmic resistance. Thus the portions adjacent to the anchorings which do not move much may with advantage be thickened-or widened or both together. The result of such procedure is to obtain what amounts to an isthmus with properties varying with the proportions adopted and determinable by mathematics or by experiment.

In this way the efiective resistance is con-.

centrated on the reduced or restricted section where-the motion incidental to vibration is greatest. It will generally be advantageous to shape the polar extensions to correspond so as to obtain the greatest possible concentratign of magnetic field where most required.

According to the fourth part of my inven tionthe natural vibrations of the recording element are damped by means of the valve supplying current thereto. This can only relatively to the'power expended in the coni be made really effective by the use of very intense magnetic fields and very low effective resistances of conduction strip. It is more easy to obtain useful damp ng in this way i when the natural frequency of the parts is tuting' a a spring not excessively high. 1 may supplement the electro-magnetic damping by mechanical damping as hereinafter set forth.

In the accompanying drawings Figures 1 to 4 are diagrammatic views showing the general principles of the in-' vention. a

Figures 5 to 21 show different arrangements of the conductor which operates the stylus and its immediate connections.

Figures 22 and 26 show forms of the complete apparatus. Figures 1, 2, 3 and 4, illustrate in diagram my invention and more particularly the first and main feature which consists in immersing the portion of the conductor\ which operates the stylus and its return portion in a magnetic field of uniform density or in parts of the same or different fiel s of thesame densit The stylus 3 which engraves the record 4 is mounted with its connections upon a block of ebonite or other suitable material which is pivoted at '66 about some suitable fixed support 125, the structure thus constl- The weight of the beam is partly borne by a smooth ball or shoe 89 which rests upon the wax record and is carried adjustably by a screw 90 to regulate the depth of cut. A1- ternativel this depth may be determined by the we I known gravity method. According to, my invention the conductor 1 is bentback upon itself so as to form with its return part 306 a U shaped strip the ends of which are attached by. soldering or other means to conducting plates or strips 107, 109, of copper, aluminium or other suitable metal. These plates lead to the ring which forms the secondary circuit of the transformer which delivers the electric current for operating the conductor .1. For clearness the arrangement of the conductor is shown'separately in Figures 2 and 3, the separation between the conductors being somewhat exaggerated. The primary of the transformer is not shown. The conductors 107, 109, are separated by insulating material 111 which may be mica or ebonite, and the whole assemblage is bolted as at'307 to the body 65. Tension may be put on the U shaped strip 1, 306, b a lever 83 having art 308. T is lever is pivoted at 84 an adjusted by a screw 8%. A suitable method of connecting the U shaped strip to thelever istorthread it through a slot 309 inthe end of the lever as shown inFigure 4. The whole or. substantially the whole of the strip 1, 306, is situated between the poles 9, 10, of a magnet (Figure 3). One of these poles is also shown by dotted lines m Figure 1." Both poles are on the sameside of the record. In the case shown, the magnetic field, or the part which matters, is of uniform. density, but this is not the case the a slot in the balance weight and the end of this leveris forked or slotted to provide for the insertion of a pin 311. The'other end is anchored also to a slotted lever 310 with a pin 312 and the coil is strained between the two pins. Either the movable lever or the fixed anchorage or both may have a certain degree of flexibility to accommodate the very slight motion of the ends of the vibrating portion due to fiexure rather than complete immobility. Both these elastic portions ma be damped with thick grease. As shown he recorder is engaging with a eating beam or balance, weight. disk blank 28 for the purpose of engraving a hill and dale record. The sliding ball and p its adjustment andalso the field ma nets are omitted for simplicit Twisted eads 313 convey current from t 0 input terminals 313 to the winding 1, 306; It is preferable.

to stick theseleads-down to the various surfaces.

A form of the invention suitable for recording a lateral cut record is indicated diagrammatically in Figure 6. The strips 1 and- 306 are arranged in bifilar fashion and strained between the anchorings 309, which are metallic studs insulated the one from the other, and the stud 81 which is the movable stud and is controlled in similar fashion to that shown in the foregoing figures by a lever and micrometer screw. The field magnet pole piecesare' indicated by dotted lines at 9 and 10, and the two branches of the conductor. 1 oscillate in opposite phase, as

in bifilar suspension perpendicularly to theplane ofthe paper. The compensatmgfporxed ball' either runs immediately beneath the magnetic air gap which then has to be made wide enough to permit of this or the lever and ball are located in the space between the polar surface and the wax blank.- Another method is to drill or cut away one of, the pole pieces to provide the necessary room. Instead of making the bifilar movement from a single conductor it may be built up from a bundle of insulated conductors forming a high resistance winding.

Figure 7 is concerned with the field magnet and its relation to the balance weight and the blank to be recorded. The polar extensions 9 and 10 are braced together by brass plates one of which is shown at 314. The yoke of the magnet is bushed by rubber 315 to provide a cushion support on the bearers 316 which may be cemented into a brick wall or pillar. A. very high intensity of magnetism 1n the air ap is aimed at and the concentrated field is more especially brought to bear on the region where the greater detection occurs. By using the strongestpossible field thevalue of the electromagnetic damping is increased. The illustration. shows the poles 9 and 10, the disk blank 28 being rotated beneath the recorder. The pivots of the balance weight 65 are in the line 66. The sliding ball is out in side elevation a form of bifilar moveshown at 89 and the micrometer ad ustment at 90. a

. F i ures 6 to 13 are diagrams indicating possible modifications and methods of workmg. Thus in Figure 8 the recording ele ment 1 is partly corrugated and partly in the form of a lever pivoting about the point 1 where it joins on to the compensating portion 306 which is fixed to the block 65. Crosses indicate the direction of the magnetic field, the poles being out of the plane of the paper. 11 Figure 9 the compensating portion is in the form of a tube and the electrical and mechanical connection of the elements 1 and 306 is by means or a small metallic diaphragm 317, about whlch the element 1v oscillates as in the case of Figure 8. Any form of damping material 318, such, for instance, as oil, petroleum jelly, wool, felt, sawdust, rubber inthe form of a bush, pellets or spon ma be'placed in the space between the tu an the yibrating element 1, as indicated by the horizontal shading. A form of instrument in WhlCll the poles are removed further from the surface ofthe wax tablet is shown in Figure 10. Here a bifilar operating conductor 1 1s connected to a stylus rocker 319 by cords or links 320 passing down the holes 321. The

stylus rocker rests'on and is pressed against,

two pivot points, one of which is shown at 322. 324 is a damping chamber nd a glass cover plate 323 renders it total y enclosed and the oil contained in it'also fills the holes 321 and is retained by caplllarity. compensating conductor is beddeddown on the ebonite and is of greater section than the working strips 1.

Figures 11 and 12 are diagrams setting ment suitable for lateral recording. The rocker 319 is connected by a tors1on wire 325 The to the straining lever 83 and the wires are stretched over the fixed peg 81 to the fixed. ring 85. Figure 12 is an explanatory plan view showing the conductors illustrated. in

306 are connected by conducting members indicated by the numeral in Figure 12,

four forming a closed circuit which is situaled between the poles 9 and 10 of a field magnet having a laminated core.-- The leads for the input which operates the conductor 1 are connected to a winding 11 which may be the field magnet Winding; This magnet in conjunction with the circuit 1, 85, 306, thus also constitutes a transformer, the winding 11 being the primary and the circuit the secondary. The field magnet battery leads are provided with suitable chokes 12. The field strength acting on the conductor 306 is sub-. stantlally the same as that acting on the portion 1. Means for stretching thenconductor 1 are provided by the screw 82, and the interspace iboundedby ring circuit 306, 85, 1 may be filled in with ebonite 326 and partly with damping material 318 which latter may take the form of a brush. 65 indigatfes the floating beam and 66 its pivot-as e ore.

lULl

Figure 16 shows a sblenoidal windingi the primary detector, the recorder, and the reproducing system are very desirable and formpart of the equipment. Such circuits are described in British specifications Nos.

160,223, 220,420 and, 190 4011110 1 others. An elaborate equipment would also include rejectors for dealing witlrany troublesome resonances however arising. The natural frequency of the apparatus may be set to approximate to 750 cycles a second or in that region and full correction made for this tone and its damping. In numbering the various details I have taken account as far as practicable of the number allotted to similar details in my previous specifications.

Figure 17 indicates the vibrating element 1 partly corrugated, also theend 340; 5:611; portion 306 is shown enlarged to provide ad ditional area of contact for the screw ur solder securing the strip. The conductor 306 is attached to or forms part of the return conductor 109 and the element 1 is connected to'air extension 107. The parts 107 and 109 are separated by. mica or like insulation 111. This figure deals with apparatus for producing hill and dale records.

Figure 18 is a fragmentary section through the members 1 and 306 of a device applying the invention to the production of needle cut disks by means of a bell crank lever 341 pivoted on an axis 342-to'a pair of bosses 343 formed on the detail 306 and actuating a cutting stylus 3. Between the corrugated strip 1 and the bell crank lever 341 is a pad of shellac 344 or other coupling means for the purpose of transferring the vibration of the strip to the lever. I may employ bell crank levers of varying ratios of mechanical magnification according to circumstances. In place of the method of pivot points or bearing illustrated, a pair .of metallic studs or wires or a leaf may be substituted so as to form an elastic hinge capable of taking up the slight lateral drift consequent on the angular displacement of the bell crank lever when in vibration. Figure 19 shows the operative conductor 1 reduced in section at the centralregion by a gradually restricted width as shown at 399. If this conductor is of aluminium it may befixedto the bars denoted 306 and 107 in Figure 19, the current being supplied by means of small screws as at 8,3; otherwise it may be soldered. It is to be understood that this conductor may be stretched by any well known means and used as in the foregoing drawings. The polar extensions are curved to the configuration of the operative conductor.

Figure 20 provides for a diminished depth of'conductor with or without any alteration advisable to laminate the field magnet and to design it on a much smaller scale. On this system there would be a considerable amount of self induction and this would have to be taken account of as affecting the characteristic response.

The principle of grading the section of the conductor either independently or in association with the configuration of the magnetic poles is not only applicable to the par- "tieular strips shown in the illustrations but applies generally to the apparatus dealt with in this specification and to all electrodynamic equipment. The grading of sunple stripsmay so alter theresilient properties of the strips that corrugations or other deformations of the material may be desir able to counteract the effects.

Although a graded strip or structure is particularly suitable for operation with a high natural frequency of ,the moving parts I do'not restrict myself'to any particular natural frequency but may use frequencies of 7000, 6000,5000, 4000, 3000, 2000, 1000,

500, 250, or intermediately. Naturally the properties of a recorder vary with the proportions' of the essential constants. More-' over, in a recording system the constants of every link in the process are involved including the tones of the recording studio. This being so an elaborate equipment would include means for correcting for the tone or period of,vibration of the recorder, means for generally. tilting the frequency scale, means for superposing integrations or differentiations of the original potentials derived from the primary detector, blunt rejector or acceptor circuits, rejector and acceptor circuits in general for dealing with casual resonances, corrections for the type of sound box to be used in reproduction, high pass and low pass filters for terminating the scale abruptly. When a brilliant record of the nature of-the pressure wave in the air is aimed at, either a suitable microphone is chosen or the effect obtained by differentiation. Such a record is suitable for a large diaphragm sound box or otherintegrating device. The properties of transformers are available'for modifying the scale characteristic especially as a means for omitting the lowest frequencies which limit the volumewithout contributing much to the total effect.

Figure 21 illustrates the application of damping to the operative strip by petroleum jelly 318 applied in the vicinity of the auchorage and the method of absorbing the energy of the natural vibration by means of a cord 400, which may be stranded metal cable, threaded through an attachment 401 to the strip and knotted as shown at 401 and leading either to a loose coil or simply secured to a distant support. Vibration travels from the element and attachments along the cord in the form of a ripple which is generally absorbed without reflection. By using a comparatively dense twisted cable and a fluify covering the absorption of energy is facilitated. This method of damping in general has been wholly or in part described and claimed-in mv British specification No. 31818 of 1925. ing the ripple absorbing cord, strip or sheet, to the acoustic element as shown, the ,anehoring 93 of the conductor 1 to the conduc In place of attachtion with commercial phonograplhs found necessary to prevent t the feed carriage I or the tor 306 maybe articulated or made in the form of a hinge and the strip continued 71ndefinitely on the side remotefrom the stylus so that ripples generated in the conductor 1 are lost in applied damping material, such as a wool covering wound on or applied to the indefinitely continued strip.

Figures 22 and 23 show an electromagnetic movement constructed according to the invention, Figure 23 being a plan view from underneath of Figure 22. The corrugated strip 1 is stretched or placed between the conducting extension 107 and the compensating conductor 306, itself forming part of the return conductor 109, theparts 107- and 109 being insulated. the one from the other by mica 111 and soldered to the ends of the secondary, winding 85 of the transformer with core 88 and primary winding 141. The secondary winding 85 is formed ofone or more turns of copper strip on edge forming a s iral or volute. In this particular design if desired this secondary winding may be approximatelyv square and used in conjunctioifwith a'core to correspond. An ebonite block with bolt 333 and screws serves to clamp the whole together, while a block 334 bushed with rubber 335 and a metallic spindle 336 forms the axis of ivoting- 66. The sensitive element 1 is strained between two screws 81 and' a tag 337 may be left for the purpose of inserting a hook into a hole 337* whereby tension may be put on the element previous to clamping down or soldering imposition. The fixing done, this tag may be clipped ofi'. Damping com osi-.

tion 318, which may consist of petro eum jelly, is inserted between the element 1 and the compensating conductor 306. By making the strip sufliciently short, for instance, about five eighths of an inch long. a natural period of some thousands of vibrations second can be obtained without any difficulty.

In the plan view a dotted trace of the field magnet poles 9 and 10 and a bracket 114 supporting the transformer core are shown. It will be seen that the conducting section 107 makes contact with the inner convolution of the secondary winding 85 conventionally denoted by portions of concentric circles.

When complete the Space between the con-.

volutions is filled in with melted shellac and the whole assemblage forms a com site floating beam. A magnetic shield 338 is also indicated by dotted lines and is Supported on the field magnet by brass distance ieces or studs 339. This shield is a usefu feature when theapparatus is used in conjuncand it'is e magnetic leakage from interfering with the motion of erally speaking it is convenient and advane tageousto operate -a recorder of this class by mean's'oftwo stage transformations, that equivalent. Genturns is to say, thatthe currents from the amplifier are stepped down by means of one transformer and again 1 transformed at 'the instrument itself. The usual sliding ball or shoe which regulates the depth of cutand carries most of the weight of the floatln beam on the wax blank has been (mi in these illustrations. 3

Figure 24 shows the field magnet having polar extensions 9 and 10 attached to the electromagnet cores 7 which latter are strengthened by the bracing 314.

Figure 25 shows the general lay out of an apparatus constructed according to my invention. Particular forms, such as" those shownin Figures 22 and 23 or in Figure 26, may be supposed to be'embodied in it.

The field magnet, one of whose oles is shownat 10, is' clamped betweena racing member 314 hinged on a pin 384 and a housing 114 designe to support the transformer core 88. The pin is held in the vertical member 385 mounted on a shaft supported in bearings 387. A lever 388 rips the shaft and serves totwist the who e apparatus in the bearing for adjustment b{ means of the screw gear389. On top of t e member 385 is a further screw adjustment 390 engaging with the magnet structure by means of a flexible joint 391 such as a ball and socket joint, which arrangement permits the magnet structure to be raised or lowerg drelatively to the revolving blank,4. The blank is driven bya phonograph 392 which is 'itself Supported ,on a cast iron base plate 393. Magnetic efiects areshielded from the metal of the phonograph by a magnetic shield 338 projecting out from the brass studs 339. A s immer is provided for the field coils.

The transformer winding 141 is. wound on dotted and denoted by 397. Other methods.

'for dealing with the problem include short circuiting the secondary winding by a large To prevent inductance but of low resistance; in suchcircumstances one may insert a condenser and sofpr'event any direct current in the sec ondary winding. A brass lever 398 serves as a' mea-ns for lowering the balanceiveight on to the surface of the blank. Figure 26 is a diagram showing the recorder and associated details, the general position of which is set out in Fi re 25. The details comprise a transfiorfii er ith laminated core 88, a primary winding tion-wound 141 located on an ebonite bin 402, a secondary winding 85 of a few of copper strip on edge insulated and bonded 5y melted shellac forming part of a composite floating beam pivoted about an v and 306, both are a block of brass 334 attached to the secondary winding and hushed with rubber 335 through which the spindle 336 passes, a

' me 73, an ebonite bracing. block counteipo' bon' mg the parts 107 and .109 which are extensions of the secondary winding insu- 1 condenser 196, low tension bus bars 169 and 170, eartln plate 173, high tension bus bars 171 and 171'. Obviously intermediate valves may be added also with correction devices corresponding to 407, 406', and 409,

and other circuital arrangements. By arlated the one from the other by mica 111,- ranging that the ratio of transformation of the workin conductor 1, and the compensating con uctor 306, which latter is integral with the extension ,109. The workin conductor is. here shown corrugated, sti ened in the centre by dishing or embossing, and connected with a stylus bar 3. Supplementary damping materialigihelie shown as at 318 and may be Vaseline or'felt or both. This material is for the purpose of checking overtones which are not effectively dealt with by the electrical damping provided by the resistance of the power, valve circuit. At is shown the trace of the polar region on either side of the parts 1 olesbeing on the same side of the record b ank. The conductor 1 is tensioned or regulated by. the locked block 405 and screw adjustment 82. Asliding shoe;8 l takes the weight of the beam on the smotih surface of the blank. This shoe is carried on a sprin'gy arm with grease damping in the slot 412. With this arrangement of-operatin and return conductors wholly imme in the magnetic field, forces due to the action of the telephonic currents are completely or substantially annulled since they act in opposin directions and so the floatin beam and a so the'magnet structure are reheved'from their influence Thi7 at it prevents sudden changes in the value of the power valve anodecurrentfrom producing any resultant force on theibeam.

Connectedwith the recorder is shown the general amplifying valve system which of course varies accordin to circumstances, it

recordin that the wave form and magnitude of t e'potentials derived from the mi:

us from the in ut transformer 406 the -ductor generates currents in the secondary crophone can be. controlled either during the recording process or otherwise. Asthe devices for this purpose are numerous their sence is rherelyindicated by one example.

incoming potentia s from the microphone act on valve 44 in the plat e circuit of which are an inductance 408, a resistance 407 and a condenser 409, which combination gives at will control over the frequency scale, that is to 80.),3110WS either the high frequency har- "1111011108 or the low frequency harmonics'to predominate. ,The Inductance, resistance and capacity are of course variable to any extent necessary." The resulting potential 7 controls valve .46'and thus the power valve 47 via the intervalve transformer 410; Other details are :-microphone pads 406, polarizing batteries as at.158,'-gr1d leak 162, grid the recorder transformer 141,- K is sufliciently. great, the resistance of the secondary microphone but for high grade work one of aperiodic type is desirable. Electro-magnetic microphones constructed according to' British specifications Nos. 160223, 222420 and 224936 (S kes and Marconi Company) are suitable. na'smuch as the control of the recording element is for the most part elastic, or even entirely elastic, the response to current is nominally proportional except near to or above the resonance point. This fact should be borne in mind in comparing this recorder with the type in which the frequency rapge to be recorded lies almost wholl above the natural frequency of the recor mg element relative to its support. Practically speakin his means a change of two difierentials. Hence a recorder ielding a characteristic response more or is of considerable practical importance since ess corresponding to the pressure wave will mean'reeord this wave in terms of displacement on the wax and such a record may be toocircuitof the transformer which tend to resist the motion. The resistance of the primary circuit multiplied by the square of the/ step up ratio gives the resistance of the pri I mary as transferred to the plate circuit..- If this is equal to or greater than that of th internal impedance-of the valve the dampi g imposed by the valve is becoming effective.

The recording stri may be stretched up (to a value nearly equa to the breaking stren th re of th'material and in such a case the uencyfmay be as high or higher than any requency of importance in music or speech.

but of course with a sacrifice of-sensitivemess. The natural frequency chosen is a matter for discretion. One merit of the compensating conductor in balance weight constructions is the prevention of undue disturbance from unsymmetrical action of the valve characteristics and especially grid current effects.

WVhat I claim is 1. An elect-ro-dynamic sound recording device comprising a stylus, a working conductor in operative connection with the said stylus, a compensating conductor in series with the working conductor, a floating beam carrying the said conductors, and means for producing a magnetic field, both conductors being immersed in the said field and in parts of the field of substantially the same strength.

2. In an electro-dynarnic sound recording device, a field magnet having its poles at one and the same side of the record blank, a stylus, a working conductor in operative connection with the said stylus, a compensatconductor, and a floating beam carrying ing conductor in series with the working both conductors, said compensating conduc tor being adapted to annul mechanical forces set up electromagnetically in the magnetic structure or the floating beam.

3. In an electro-dynamic recording device as claimed in claim 2, a field electro-magnet between the. poles of which the two said conductors are arranged, thecompensating conductor being adapted to protect the magnetic structure and the floating beam from mechanical forces arising from currents in the conductor.

4. Apparatus as claimed in claim 1, in which the working conductor consists of a thin corrugated sheet.

5. In apparatus as claimed in claim. 1, means for damping the o erative conductor.

In testimony that I c aim the foregoing as my invention I have signed my name this 5th day of August, 1926.

ADRIAN FRANCIS SYKES. 

